The present invention relates to a method and a device for the analysis and quantification of flows, in particular for the three-dimensional determination of flow velocity components or of the three-dimensional visualization of flows in fluids or gases.
Measuring flow velocities and visualizing flows have broad applications, especially in aerodynamics and in fluid dynamics, in the analysis and optimization of the most varied flow phenomena, as well as in the area of industrial process engineering and production technology. In this context, mechanical, electromechanical, as well as optical flow measuring methods are used. The existing optical flow measuring methods, in this regard, can be roughly subdivided into point, surface, and spatial measuring methods.
In this way, the surface measurement of intermittent flow processes or of spatial turbulence structures has been possible heretofore using so-called total-field methods. These methods detect in liquid or gas flows the scattered light of particles suspended therein in light sections or a light-section sheets.
In addition, in the case of surface measuring methods, so-called xe2x80x9cParticle Image Velocimetry (PIV)xe2x80x9d and the xe2x80x9cParticle Tracking Methodxe2x80x9d are widely used. In this context, the shift of suspended particle groups or of individual particles that are suspended in a medium that is to be analyzed is determined with respect to the flow conditions using correlation, or tracking, algorithms.
In addition, in surface measuring methods, it is known to use two different, colored light sections, at the same time, for determining the normal velocity components of the suspended particles perpendicular to the light-section sheets. In this regard, reference should be made, by way of example, to I. Kimura and Y. Kohno, xe2x80x9cMeasurement of Three-dimensional Velocity Vectors In a Flow Field Based on Spatio-Temporal Image Correlation,xe2x80x9d 3rd International Symposium FLUCOME, pp. 609-615, (1991), C. Brucker, xe2x80x9c3-D PIV Via Spatial Correlation in a Color-Coded Light-Sheet,xe2x80x9d Experiments in Fluids, 21, pp. 312-314, Springer Publishing House, 1996, and A. Cenedese and A. Paglialunga, xe2x80x9cA New Technique For the Determination of the Third Velocity Component with PIV,xe2x80x9d Experiments in Fluids, 8, pp. 228-230, Springer Publishing House, 1998.
In M. Raffel et al., xe2x80x9cAnalytical and Experimental Investigations Of Dual-Plane Particle Image Velocimetry,xe2x80x9d Optical Engineering 35, 7, pp. 2067-2074, (1996), the further suggestion is made to spatially transpose a single light section into two light-section positions using a chopper disk.
Finally, from F. Dinkelacker et al., xe2x80x9cDetermination of the Third Velocity Component with PTA Using an Intensity Graded Light Sheet,xe2x80x9d Experiments in Fluids 13, pp. 357-359, Springer Publishing House, 1992, it is already known to modulate the intensity of individual thicker light sections along a light section depth.
Summarizing, the cited surface methods make it possible to determine the velocity components of the suspended particles within a plane and therefore also to analyze the flows in the fluid or gas to be examined. However, they are only capable of analyzing three-dimensional flows in one plane and not in a volume.
Among the spatial measuring methods, i.e., those measuring methods which permit the analysis of flows in a volume, stereoscopic methods should be mentioned, which are known, by way of example, from R. Racca and J. Dewey, xe2x80x9cA Method for Automatic Particle Tracking in a Three-Dimensional Flow Field,xe2x80x9d Experiments in Fluids 6, pp. 25-32, Springer Publishing House, 1988, or which function using stereoscopic lenses, as proposed by T. Chang et al., xe2x80x9cApplication of Image Processing to the Analysis Of Three-Dimensional Flow Fields,xe2x80x9d Optical Engineering, 23, 3, pp. 282-287, (1984). In this method, the flow field is recorded from different directions using two to four cameras.
All of the above-mentioned spatial measuring methods have in common that they have a continual illumination of the flow field to be analyzed and/or that the volume to be analyzed is recorded from different directions using a plurality of image detectors. Therefore, these methods are only partially applicable for practice where setup times, optical accessibility, and limitations regarding direction of observation play an important role. The latter, furthermore, also applies to holographic methods.
Finally, from C. Brxc3xccker, xe2x80x9cDigital-Particle-Image-Velocimetry (DPIV) in a Scanning Light Sheet: 3-D Starting Flow Around a Short Cylinder,xe2x80x9d Experiments in Fluids 19, pp. 255-263, Springer Publishing House, (1995), a spatial measuring method is known, in which the volume to be analyzed is scanned using a drum scanner having a monochromatic laser beam. In this context, the scattered light characterizing the flow and scattered in the suspended particles is recorded as a function of time using a high-speed camera. For this purpose, each individual light-sheet position in the volume to be analyzed is separately recorded, so that the recording of the flow field is tied to the image repetition frequency of the camera used. In addition, the separate recording of each individual light-sheet position in the detection space generates a very large quantity of data having correspondingly large memory requirements.
The objective of the present invention is to carry out the measurement of flow velocities and the analysis of flows in gases and liquids within a detection space in a three-dimensional manner and at the same time more simply, more rapidly, and more cost-effectively.
In contrast to the related art, the method and the device according to the present invention have the advantage of relatively small equipment expense, especially with regard to the detection device. Furthermore, it is advantageous that only one observation direction, i.e., only one CCD color camera, is required.
In addition, the method according to the present invention has the advantage that the data sets that arise are relatively small, and that they therefore can be processed and evaluated easily and straightforwardly.
Finally, the attainable resolution, i.e., measuring precision, in the method according to the present invention is now no longer tied, for example, to the image repetition frequency of a high-speed camera, but is only limited by the distance and the temporal difference in the generation of two adjacent, parallel light sheets which are arranged in spatial succession.
Therefore, it is particularly advantageous if a multiplicity of light sheets having light of different colors or different frequency spectrums are used, these colors potentially lying both in the visible frequency range as well as in the near ultraviolet or near infrared range. In this case, for recording within the detection space the light that is scattered by or emanating from the particle characterizing the flow, a conventional and therefore relatively economical CCD color camera is suitable.
Suitable as the electromagnetic waves, or light, is, on the one hand, a polychromatic light beam, polychromatic here being understood to be a light beam which covers a wider frequency spectrum in the visible frequency range and appears, for example, to the human eye as white or as a secondary color, and, on the other hand, if appropriate, a plurality of light beams of this type, which make available in each case one or a plurality of different colors, i.e., primary colors.
In this context, the light source for this or these light beams can be one or a plurality of lasers or an arrangement of laser diodes, which, if necessary, each generate different colors, secondary or primary colors (red/yellow/blue). In addition, projection lamps having point-plotting light surfaces are also used.
Particularly advantageous are one or a plurality of polychromatic laser beams, because in this manner a particularly good collimation and spatial resolution, i.e., separation, of the individual light sheets in the detection space is achieved.
To assure that, in the raster scanning of the detection space through the parallel light sheets at the location of the image detectors, i.e., of the detection device, the depth of focus produced is good and always at least substantially consistent, it is advantageous if the detection device or the CCD color camera used is provided with an additional device for the continuous or step-by-step adjustment of the depth of focus. In this context, the adjustment of the depth of focus, for example, using a control unit, is correlated with the raster scanning of the detection space via the light sheets that are generated in temporal succession.
Well suited to evaluate the images of the detection space recorded by the CCD color camera, i.e., the detection device, are generally known algorithms and evaluation methods from xe2x80x9cParticle Image Velocimetry,xe2x80x9d which additionally take into account the color information. However, xe2x80x9cParticle Tracking Methodsxe2x80x9d can also be used.
Overall, in the aforementioned methods known to the worker skilled in the art, it is only necessary to expand them with respect to color recognition or frequency or frequency band recognition and with respect to the evaluation of the frequency or color information, for quantifying the normal velocity components.
A simple and rapid filtering of the polychromatic light made available by the light source is advantageously carried out using a generally known acoustooptic modulator, which makes possible a color mixing, i.e., the generation of any and all colors, at a color change frequency that extends into the MHZ range.
In addition, it is advantageous to provide in the illuminating device a collimator and a polygon scanner having an attached galvanometer scanner, which make it possible to raster scan the detection space at a high spatial resolution, i.e., at minimal width and clearer spatial separation of the individual adjoining light sheets.